1. Field of the Invention
The present invention relates to a novel positive resist composition, and a method of forming a resist pattern using the same.
The application claims priority from Japanese Patent Application No. 2008-161634 filed on Jun. 20, 2008, and Japanese Patent Application No. 2008-247802 filed on Sep. 26, 2008, the disclosures of which are incorporated by reference herein.
2. Description of the Related Art
In recent years, in the production of semiconductor elements and liquid crystal display elements, advances in lithography techniques have led to rapid progress in the field of pattern miniaturization.
Typically, these miniaturization techniques involve shortening the wavelength of the exposure light source. Conventionally, ultraviolet radiation typified by g-line and i-line radiation has been used, but nowadays KrF excimer lasers and ArF excimer lasers are now starting to be introduced in mass production of semiconductor elements. Furthermore, research is also being conducted into lithography techniques that use exposure light source having a wavelength shorter than these excimer lasers, such as F2 excimer lasers, electron beam, extreme ultraviolet radiation (EUV), and X ray.
As shortening of the wavelength of the exposure light source progresses, it is required to improve various lithography properties of the resist material, such as the sensitivity to the exposure light source and a resolution capable of reproducing patterns of minute dimensions. As a resist material which satisfies these conditions, a chemically amplified resist is known, which includes a base material component that exhibits a changed solubility in an alkali developing solution under action of acid and an acid generator component that generates acid upon exposure. These chemically amplified resists include negative resists in which the alkali solubility decreases upon exposure, and positive resists in which the alkali solubility increases upon exposure.
Conventionally, polymers have been used as the base material components within these types of chemically amplified resists, and examples of these polymers include polyhydroxystyrene (PHS), PHS-based resins in which a portion of the hydroxyl groups of a PHS have been protected with acid dissociable, dissolution inhibiting groups, copolymers derived from (meth)acrylate esters, and resins in which a portion of the carboxyl groups within these (meth)acrylate esters have been protected with acid dissociable, dissolution inhibiting groups.
However, when a pattern is formed using these types of pattern-forming materials, a problem arises in that roughness can develop on the upper surface and side wall surfaces of the pattern. For example, roughness on the side wall surfaces of a resist pattern, so-called line edge roughness (LER), can cause distortions around the holes in hole patterns, and fluctuations in the line width in line and space patterns, and consequently has the potential to adversely affect the formation of very fine semiconductor elements.
This problem becomes more significant as the pattern dimensions are reduced. Accordingly, in lithography processes using electron beams or EUV or the like, which are targeting the formation of very fine patterns with dimensions of several dozen nm, very low levels of roughness that are superior to current levels of pattern roughness are being demanded.
However, the polymers typically used as base material components have a large molecular size (or root mean squared radius per molecule) of several nm. In the developing step of a pattern formation process, the solubility behavior of the resist with respect to the developing solution typically occurs in single molecule units of the base material component, meaning that as long as polymers are used as the base material component, further reductions in the level of roughness will remain extremely difficult to achieve.
In order to overcome this type of problem, resists that employ a low molecular weight material as the base material component have been proposed as potential materials for achieving ultra low levels of roughness. For example, Non-Patent Documents 1 and 2 propose low molecular weight materials having alkali-soluble groups such as hydroxyl groups or carboxyl groups, in which some or all of these groups have been protected with acid dissociable, dissolution inhibiting groups.    [Non-Patent Document 1] T. Hirayama, D. Shiono, H. Hada and J. Onodera: J. Photopolym. Sci. Technol., 17 (2004), p. 435    [Non-Patent Document 2] Jim-Baek Kim, Hyo-Jin Yun, Young-Gil Kwon: Chemistry Letters (2002), pp. 1064 to 1065.